Hybrid vehicles have recently been of great interest as environment-friendly vehicles. The hybrid vehicles are now partially commercialized.
A hybrid vehicle has, as its motive power sources, a DC (direct current) power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. More specifically, the engine is driven to secure the motive power source and a DC voltage from the DC power supply is converted by the inverter into an AC (alternating current) voltage to be used for rotating the motor and thereby securing the motive power source as well.
Regarding the hybrid vehicle, it has been proposed to boost the DC voltage from the DC power supply with a voltage step-up converter and supply the boosted DC voltage to the inverter which drives the motor (Japanese Patent Laying-Open No. 8-214592).
The voltage step-up converter is comprised of two NPN transistors connected in series between a power supply line and a ground line of the inverter and a reactor having one end connected to an intermediate point between the two NPN transistors and the other end connected to a power supply line of the power supply.
In the voltage step-up converter, the NPN transistor connected to the power supply line (upper arm) and the NPN transistor connected to the ground line (lower arm) are turned on/off at a predetermined duty ratio for boosting the DC voltage from the power supply to provide the boosted voltage to the inverter and for decreasing a DC voltage from the inverter to provide the decreased voltage to the power supply.
In view of the fact that the upper arm and the lower arm constituting the voltage step-up converter are connected in series between the power supply line and the ground line, it is necessary to prevent the upper arm and the lower arm from being simultaneously turned on. Then, control signals for controlling switching of the upper arm and the lower arm each include a dead time for preventing the simultaneous turn-on of the upper arm and the lower arm.
FIG. 7 is a timing chart of control signals controlling the upper arm and the lower arm. Referring to FIG. 7, the upper arm and the lower arm are turned on/off at a predetermined duty ratio in each control period T. If the upper arm having been turned off and the lower arm having been turned on are turned on and off respectively at timing t1, the upper arm and the lower arm could simultaneously be in the “on” state. Therefore, the lower arm is turned off at timing t1 and thereafter the upper arm is turned on at timing t2 later than timing t1 by a certain dead time.
When a voltage command value of the voltage step-up converter is considerably close to a power supply voltage, the on-duty of the upper arm (referring to the “on” period of the upper arm) is fairly high, 0.98 for example. In such a case, the on-duty of 0.98 is partially occupied, namely shortened by the dead time, and thus the period of time during which the upper arm should be in the on state cannot be secured.
FIGS. 8A and 8B are timing charts of the voltage and the on-duty of the upper arm. Referring to FIGS. 8A and 8B, it is supposed here that boosting of the voltage initially at power supply voltage Vb is started at timing t0. The voltage is accordingly increased from power supply voltage Vb. In the period from timing t0 to timing t1, the voltage command value is considerably close to power supply voltage Vb, so that the on-duty of the upper arm that is calculated based on the voltage command value is partially occupied by the dead time of the upper arm and the originally intended on-duty cannot be secured. Therefore, the on-duty of the upper arm is not controlled in a linear manner in the range between 1.0 and 0.95 and consequently oscillates (see FIG. 8B). Accordingly, the output voltage of the voltage step-up converter also oscillates (see FIG. 8A).
Then, when the on-duty of the upper arm that is calculated based on the voltage command value reaches for example 0.95, the on-duty is not partially occupied by the dead time and thus can be controlled in the linear manner.
As seen from the above, in the region where the voltage command value is considerably close to power supply voltage Vb, the on-duty of the upper arm is partially occupied by the dead time, so that the output voltage of the voltage step-up converter oscillates and accordingly the DC current from the power supply also oscillates. This could result in breakage of the power supply.